When letters, patterns, images, or the like (which are hereinafter referred to generically as “printings” if necessary) are printed, for example, on a non-absorptive surface of a piece of metal such as aluminum foil or the bottom lid or top lid of a can; a plastic film; a sheet of coated paper; or a sheet of laminated paper according to an inkjet printing method, it has heretofore been a common practice to heat and dry the printings that have been printed thereon.
However, recently, an inkjet ink, such as HEATLESS INK (registered trademark), has been developed and been put to practical use in which a heating/drying step can be excluded by providing quick-drying properties resulting from the fact that only an organic solvent is used as a solvent or that, even when water is used together as a solvent, a so-called organic-solvent-rich state is created by using an organic solvent more abundantly than water.
Additionally, recently, various examinations have been made to develop a quick-drying inkjet ink that is capable of improving the fixability of printings with respect to a surface of a metallic object such as aluminum foil or the bottom lid or top lid of a can (hereinafter, which is referred to briefly as a “metal surface” if necessary), or a surface made of material having low polarity, such as a surface of a polyolefin-based object of which a polypropylene film or a polypropylene laminated paper (hereinafter, which is referred to briefly as a “low-polarity surface” if necessary), which has difficulty particularly in improving the fixability of printings among the non-absorptive surfaces.
Although the use of the inkjet printing method has been examined, for example, in order to print a drug name or the like on a surface of aluminum foil that forms the PTP (Press Through Package) of medicines or in order to print a mark of, for example, a production factory, a best-before date, or the like on the bottom lid or top lid of a can, however the metal surface such as the surface of the aluminum foil or the bottom lid or top lid of the can is non-absorptive so that the inkjet ink does not infiltrate into its inside, and therefore it is difficult to firmly fix printings, such as the drug name.
Therefore, for example, PTL 1 proposes to improve the adhesiveness of printings with respect to a metal surface by additionally combining a carboxylic acid that has a benzothiazole ring with a non-aqueous, quick-drying inkjet ink including a colorant, a binder resin, and an organic solvent.
PTL 2 proposes to improve the adhesiveness of printings with respect to a metal surface by selectively using a styrene-maleic acid resin as a binder resin.
However, disadvantageously, in the inkjet inks of PTL 1 and PTL 2, the effect of improving the fixability of printings with respect to the metal surface or the low-polarity surface or the like based on the improvement of adhesiveness has not yet been sufficient, and the printings are easily blurred or easily come off, for example, when the printings are repeatedly rubbed with a fingertip or the like.
Additionally, disadvantageously, in the inkjet inks of PTL 1 and PTL 2, if a decapping time becomes long, a printer nozzle will be clogged and, when printing is restarted, blurring is liable to occur in a case in which the inkjet ink is used in, for example, an on-demand type inkjet printer.
The term “decapping time” denotes a period of time during which an inkjet ink in a nozzle among nozzles provided in an on-demand type inkjet printer is continuously being exposed to the open air in a standby state in which ink droplets are not discharged in accordance with a printing pattern in intermittent printing.
Inmost cases, an on-demand type inkjet printer is normally provided with a function designed to close a nozzle (to cap a nozzle) so that clogging does not occur resulting from the fact that an inkjet ink in the nozzle is exposed to the open air and is dried while the operation of the printer is being stopped.
However, such capping is released when printing is performed, and therefore a nozzle, which is in a standby state particularly when intermittent printing is performed, remains in a state (decapped state) in which the nozzle is not closed until ink droplets are discharged next time, and the inkjet ink in the nozzle is continuously exposed to the open air during that time.
Therefore, there is a tendency for the nozzle to be clogged more easily in proportion to an increase in this period of time, i.e., in proportion to an increase in the duration of the decapping time.
Particularly in a thermal type inkjet printer in which bubbles are generated by momentarily heating an inkjet ink and in which an ink equivalent to an increase in its volume is discharged as ink droplets through a nozzle, a solvent is repeatedly volatilized because of the generation of the bubbles when printing is performed, and, in response thereto, its local concentration is repeatedly changed, and therefore there is a tendency for solids, such as a binder resin, to be easily precipitated and for a nozzle to be easily clogged therewith during the decapping time.
Hereinafter, a feature characterized in that the nozzle is not easily clogged during the decapping time is evaluated in terms of whether “intermittent printability” is excellent or not. An inkjet ink that causes clogging during a very short decapping time can be evaluated as being not excellent in intermittent printability, whereas an inkjet ink can be evaluated as becoming more excellent in intermittent printability in proportion to an increase in the duration of a decapping time during which clogging is not caused.
In the case of a medicine, its kind etc., must be reliably discriminated from another. Therefore, printings that have been printed in PTP or the like are required not only to be not easily blurred in printing because of clogging that results from the defect in intermittent printability but also to be not easily blurred or not easily come off when the printings are repeatedly rubbed with a fingertip or the like as described above.
In order to improve the fixability of printings with respect to a low-polarity surface, such as a polyolefin-based surface, for example, PTL 4 proposes to combine at least one kind that is selected from the group consisting of a terpene phenol resin, a dicyclopentadiene resin, and a rosin ester resin and that is used as a tackifier functioning to give tackiness to a polyamide resin with a non-aqueous, quick-drying inkjet ink including the polyamide resin as a binder resin.
PTL 5 proposes to concurrently use two kinds of polyamide resins that are mutually different in acid number and each of which is used as a binder resin, and proposes to combine a rosin-modified maleic acid resin and/or a terpene phenol resin that are/is used as a tackifier therewith.
Additionally, PTL 6 proposes to combine a polyamide resin that is used as a binder resin, a rosin-modified maleic acid resin that is used as a tackifier, and a titanium chelate together.
However, disadvantageously, in any of the inkjet inks of PTL 4 to PTL 6, precipitation is liable to occur because stability is insufficient during ink storage, and blurring is liable to occur when printing is performed because of clogging resulting from the aforementioned defect of intermittent printability in addition to the fact that the effect of improving the fixability of printings with respect to the aforementioned low-polarity surface or the metal surface is still unsatisfactory.
PTL 7 proposes to concurrently use a polyamide resin that is used as a binder resin and a terpene phenol resin and/or a rosin ester resin that has a specific hydroxyl value.
According to an inkjet ink in which a dye is additionally joined as a colorant in the arrangement of PTL 7 mentioned above, it is possible to more greatly improve the fixability of printings with respect specifically to a low-polarity surface, such as a polyolefin-based surface, than conventional inkjet inks, such as those of PTL 4 to PTL 6.
However, the aforementioned inkjet ink is yet insufficient in the fixability of printings with respect specifically to a metal surface, and still has a problem in the fact that the printings are easily blurred or easily come off when the printings are repeatedly rubbed with a fingertip or the like.
There is a case in which a pigment is required to be used as a colorant in an inkjet ink, i.e., for example, a white pigment, such as titanium oxide, is required to be used as a colorant in order to express white.
However, disadvantageously, in an inkjet ink in which a pigment is additionally joined as a colorant in the arrangement of PTL 7, the low-polarity surface, without being limited to the metal surface, becomes insufficient in the fixability of printings, and the printings are easily blurred or easily come off when the printings are repeatedly rubbed with a fingertip or the like.